We propose studies in three specific areas on the mechanism of action of aldosterone on sodium reabsorption and urinary acidification: (1) Studies on aldosterone-binding protein (ABP). Because of in vitro lability of ABP, isolation and purification of ABP (or aldosterone receptor) must be achieved either by a rapid method or by a means which stabilizes the aldosterone-protein complex. We will pursue rapid purification by the powerful technique of affinity chromatography and stabilization by use of the new technique, photo-affinity labelling. (2) Studies on aldosterone-induced protein (AIP). We elect two avenues of study to directly attack the question of what protein(s) is synthesized to cause aldosterone's effect on ion transport. First, we will follow up an unexpected finding of increased incorporation of H3-leucine into protein aldosterone-treated epithelial cells of toad bladder. Second, we will seek to identify the "sodium permease" by photo-affinity labelling using a unique photolyzable derivative of the diuretic, amiloride. (3) Studies on urinary acidification. We will explore several physiological and biochemical parameters to gain an understanding of the mechanism of hormonal regulation of the urinary acidification system present in the isolated toad bladder.